Which MongoDB Atlas option balances cost savings with required features for low‑traffic apps?

The goal is to lower monthly expenses for a low‑traffic application by selecting a MongoDB Atlas tier that avoids unnecessary compute charges while still meeting the application’s data access patterns.

The application relies on multi‑document transactions and the $lookup aggregation stage, features that are not available on Atlas Serverless instances. The free M0 tier offers limited storage and shared RAM, and may throttle under sustained write load, introducing uncertainty about its suitability for occasional write bursts.

Does the M0 tier provide sufficient throughput for occasional writes without throttling? Can the Serverless tier be used if the application is refactored to avoid unsupported operations? Which option offers the best cost‑feature trade‑off for this workload?